U.S. Pat. No. 4,612,555 and particularly FIG. 7 thereof discloses a control circuit for a laser beam scanner apparatus. The laser printer described therein has a switching amplifier for controlling a laser diode. A variable current source is used as a common emitter resistor for the two switching transistors and a second current source is used to adjust the base current through the laser diode. A photodiode is also provided which measures the emitted amount of light. The energization current through the laser diode is determined on the basis of this measurement of the emitted amount of light.
One characteristic of a laser diode is that when a current is supplied to it, a small period of time elapses before the laser diode actually emits light. This switch-on or turn-on delay time is particularly troublesome when the laser diode is used in a high speed or high resolution laser printer. With a switch-on delay time of about 6 nsec and a writing frequency of about 10 MHz, the switch-on point of the laser lies approximately 6 .mu.m later than expected. A broadening of the image due to this inherent time delay is visually disturbing to the eye with respect to fonts with a resolution, for example, of 20 image dots per mm.
This troublesome characteristic of an inherent turn-on delay time has not been overcome in the area of a laser diode in a laser printer. For example the Transaction of the IECE of Japan, Vol. E65, No. 10 (October 1982) at pp. 584-85 deals with the duty cycle of a diode laser in an optical disk system, not a laser printer. Similarly the Conference Proceedings of the Third European Conference On Optical Communication (Sept. 14-16, 1977) NTC-Fachberichte Band 59 at pp. 208-10 relates to optical communication systems, wherein the main requirement is that the turn-on delay be as short as possible so that the light pulses fall within the appropriate time slot. Other image recording devices are described in U.S. Pat. Nos. 4,387,983 and 4,594,596.
It would be desirable, therefore, to provide a control circuit for a laser printer using a laser diode wherein the inherent time delay as well as other disadvantages are obviated.